1. Field of the Invention
This invention relates to a fuser and an image forming apparatus.
2. Description of Related Art
With a conventional image forming apparatus, such as a photocopier, a printer, a facsimile machine, an MFP (Multi Function Peripherals), or the like, for example, with a conventional printer, a surface of a photosensitive drum is charged equally and uniformly to form an electrostatic latent image thereon upon exposure, thereby forming a toner image upon attachment of a toner on the electrostatic latent image. The toner image is transferred onto a paper and thereafter fused with a fuser to form an image.
The fuser comprises a fusing roller serving as a heating element having a built-in halogen heater, and a pressure roller coming in contact with the fusing roller in a pressing manner, in which the toner of the toner image is heated as fused with the fusing roller, thereby being pressed onto the paper with the pressure roller, and as a result, the toner image is fused onto the paper.
A thermistor is brought in contact with the fusing roller to detect a surface temperature of the fusing roller.
In a power supply circuit of the printer, a current supplied from a power supply to the halogen heater is controlled upon supplying a control signal from a CPU to a triac. The CUP generates, where receiving a sensor output transmitted from the thermistor, the control signal in a manner to set the surface temperature of the fusing roller to be such a fusing temperature as previously set, thereby supplying the control signal to the triac.
In the meanwhile, the halogen heater is to be energized only when using the printer whereas being nonenergized upon breaking the current when not using the printer to reduce energy consumed for the fuser. The printer of this type, however, employs the halogen heater as a heat source, thereby requiring a long period until when the surface temperature of the fusing roller reaches a set temperature.
Therefore, such a fuser has been provided, as heating the fusing roller upon induction heating (see, e.g., Japanese Patent Laid-Open No. H11-231697)
FIG. 2 is a cross-sectional view of the conventional fuser employing an induction heating method.
In FIG. 2, numeral 11 is a fusing roller comprising a core metal, not shown, and an elastic layer formed outside the core metal. Numeral 13 is a magnetic field generator having a core 13a, a coil 13b wound around the core 13a, and securing members 14 disposed at both ends of the core 13a. A predetermined gap is formed between the fusing roller 11 and the magnetic field generator 13, with a roller 12 rotatably disposed at each of the securing members 14.
Furthermore, numeral 15 is a pressing supporter for supporting in a shakable manner as pressing the magnetic filed generator 13 against the fusing roller 11. Numeral 16 is a spring for urging the magnetic field generator 13 toward a side of the fusing roller 11. Numeral 17 is a power supply connected to the coil 13b, for supplying the current thereto. Numeral 18 is a pressure roller for applying pressure on the fusing roller 11, for forming nip area N. Numeral 19 is a thermistor for detecting a surface temperature of the fusing roller 11. Additionally, numeral 20 is a paper.
The core metal of the fusing roller 11 functions as an induction heating element to allow an eddy current to pass therethrough in a case where the magnetic field generator 13 generates a magnetic field upon current supply to the coil 13b, and as a result, Joule heat is generated to heat the fusing roller 11.
With the conventional fuser as described above, it is necessary to retain an appropriate value of the gap between the core metal and the magnetic field generator 13, but an elastic layer formed outside the core metal, against which the roller 12 is pressed, expands with heat to undesirably push out the roller 12 in a radially outward direction in a case where a temperature becomes high.
As a result, the gap between the core metal and the magnetic field generator 13 becomes wider as the elastic layer expands, and the core metal reduces in heating value thereof to make it difficult to control the fusing roller 11 in a manner to set the surface temperature thereof to an appropriate fusing temperature.
This invention aims to solve such problems as described above and to provide a fuser and an image forming apparatus capable of controlling the heating element in a manner to set a surface temperature thereof to an appropriate fusing temperature.